null_safety rebuild script (#1369) (#1371)

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel channel on [Discord].

<!-- Links -->
[Flutter Style Guide]: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]: https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

platform_channels/android/app/build.gradle

@@ -27,6 +27,7 @@ apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
 
 android {
     compileSdkVersion flutter.compileSdkVersion
+    ndkVersion flutter.ndkVersion
 
     compileOptions {
         sourceCompatibility JavaVersion.VERSION_1_8
@@ -44,6 +45,8 @@ android {
     defaultConfig {
         // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
         applicationId "dev.flutter.platform_channels"
+        // You can update the following values to match your application needs.
+        // For more information, see: https://docs.flutter.dev/deployment/android#reviewing-the-build-configuration.
         minSdkVersion flutter.minSdkVersion
         targetSdkVersion flutter.targetSdkVersion
         versionCode flutterVersionCode.toInteger()